1. Field of the Invention
This invention relates to a spark timing control system for a spark ignition internal combustion engine, and more particularly to a spark timing control system equipped with a sensor for detecting an increase in the output power required of the engine to retard spark from the normal setting to obtain an optimal operation of the engine when an increase in the output power is required of the engine, for example, in the case of quick acceleration.
2. Description of the Prior Art
It is known to combine a knock detector with a spark timing control system for a spark ignition internal combustion engine, in order to achieve an optimal operation of the engine from the standpoint of fuel economy and knock prevention. This conventional system retards spark when knocking occurs in the engine. On the other hand, the system usually advances spark timing as long as knock is absent in the engine. Since most engines tend to experience an increased knocking level as spark timing is advanced, the system constitutes a closed-loop control of spark timing and usually controls spark timing so as to operate the engine just under the knock limit or under trace (extremely-weak) knock conditions. In general, engine operation just under the knock limit or under trace knock conditions approximately coincides with the optimal operation of the engine from the standpoint of fuel economy and knock damage to the engine.
However, such a closed-loop spark timing control system falls into difficulty in controlling spark timing properly during engine transitional conditions, such as quick acceleration, where an increase in the output power is required of the engine, because the closed-loop control involves a relatively large time-constant or response time which prevents rapid change of spark timing in response to engine knock.